A vehicle fuel system may include an evaporative emissions system designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor storage canister packed with an adsorbent that adsorbs and stores the vapors. At a later time, when the engine is in operation, the fuel vapors may be purged from the evaporative emissions system into an engine intake manifold for use as fuel. In some examples, the evaporative emissions system may be configured to store refueling vapors, running loss vapor, and diurnal vapors. However, in other examples, the evaporative emissions system and fuel system may be configured as a non-integrated refueling canister only system (NIRCOS). In such a system, the fuel tank is sealed, and fuel vapors are only routed to the fuel vapor storage canister during a refueling event. For example, a plug-in hybrid electric vehicle (PHEV) may include a NIRCOS due to limited engine run time. If the fuel tank were vented, prolonged electric mode driving (in which the engine is off and the vehicle is propelled with torque from an electric motor) may result in the fuel vapor storage canister becoming overloaded and fuel vapors being emitted to the atmosphere.
In order to seal the fuel tank from the fuel vapor storage canister, the NIRCOS may include a fuel tank isolation valve (FTIV) between the fuel tank and the fuel vapor storage canister. For example, the FTIV may be a solenoid valve under control of a pulse-width modulated signal. Typically, a pressure sensor is coupled to the fuel system (such as a fuel tank pressure transducer coupled to the fuel tank) in order to measure a fuel tank pressure. An additional pressure sensor may be included on the fuel vapor storage canister-side of the FTIV in order to monitor for degradation in the evaporative emissions system. Based on the fuel tank pressure, the FTIV may be at least partially opened to vent the fuel tank during engine-on conditions and to prepare the fuel tank for refueling (e.g., during engine-off conditions). However, the inventor herein has recognized that including two pressure sensors in the NIRCOS may increase the cost of the system and lead to multiple points of degradation, as each sensor may independently degrade, and has further recognized that vehicle costs and complexity may be reduced by including a single delta pressure sensor coupled across the FTIV. The delta pressure sensor may output a signal corresponding to a pressure difference (e.g., a differential pressure) between the evaporative emissions system and the fuel system, which may be used to detect both evaporative emissions system degradation and fuel system degradation. Further, the differential pressure may be used to determine a fuel tank relative pressure. However, while the FTIV is open, the differential pressure approaches zero and therefore may not be used to determine the fuel tank relative pressure. Without feedback concerning the fuel tank relative pressure while the fuel tank is vented, the fuel tank relative pressure may be overestimated. As a result, the FTIV may be maintained open for a prolonged duration, which may delay refueling and/or increase a loading of the fuel vapor storage canister compared to when the duration is not prolonged.
In one example, the issues described above may be addressed by a method, comprising: venting a fuel tank by pulsing a fuel tank isolation valve (FTIV) open and closed in response to a depressurization condition determined based on a differential pressure measured by a delta pressure sensor coupled across the FTIV; and adjusting the pulsing responsive to the differential pressure measured while the FTIV is closed and independent of the differential pressure measured while the FTIV is open during the pulsing. In this way, a single delta pressure sensor may be used to determine whether to vent the fuel tank by pulsing the FTIV as well as to adjust the pulsing.
As one example, a first pressure port of the delta pressure sensor may be fluidically coupled to a conduit between the FTIV and the fuel tank, and a second pressure port of the delta pressure sensor may be fluidically coupled to the conduit between the FTIV and a fuel vapor storage canister of an evaporative emissions system. With no restricting components, such as valves, present between the fuel tank and the first pressure port and/or the fuel vapor storage canister and the second pressure port, the differential pressure measured by the delta pressure sensor indicates a relative pressure (or vacuum) of the evaporative emissions system as well as a relative pressure (or vacuum) of the fuel tank. With the evaporative emissions system normally vented to atmosphere, the differential pressure may indicate a pressure of the fuel tank relative to atmospheric pressure. Thus, the depressurization condition may be determined when the fuel tank relative pressure is greater than or equal to a threshold relative pressure, which may be a predetermined relative pressure value above atmospheric pressure. Further, a rate of the pulsing may be adjusted based on the fuel tank relative pressure, such as by decreasing the rate as the fuel tank relative pressure approaches the threshold relative pressure. By pulsing the FTIV open and closed and determining the relative pressure of the fuel tank only when the FTIV is closed, feedback concerning the fuel tank pressure may be maintained while the fuel tank is vented, resulting in efficient tank venting.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.